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   A simple Torque rod model that uses a tanh function to model the dipole
   curve for a typical torque rod. The initialization is
   
   u.mSaturation       - dipole at saturation
   u.saturationVoltage - the voltage at saturation
   u.mResidual         - the residual dipole

   To demo the model type:

   TorqueRod( 'initialize' );
   TorqueRod( 'set voltage', linspace(-15,15) );
   m = TorqueRod( 'get moment' );

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   Form:
   x = TorqueRod( action, u )
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   ------
   Inputs
   ------
   action        (1,:)   Actions
   u             (1,1)   Input depends on action

   -------
   Outputs
   -------
   x             (1,:)   Output depends on action

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